Apparatus for alignment of a multicore fiber in a multifiber connector and method of using same

ABSTRACT

A multicore fiber alignment apparatus is described, having a chassis into which is mounted ferrule-holding means for holding a multicore fiber ferrule having one or more capillaries extending therethrough. Fiber-holding means for holding one or more multicore fibers in position to be mounted into the ferrule, such that each multicore fiber extends through a respective ferrule capillary. Means are provided for monitoring the rotation angle of each multicore fiber within its respective capillary, relative to a reference rotational orientation. Means are further provided for rotating each of the multicore fibers within its respective capillary. The rotational orientation of each multicore fiber is fixed when its rotation angle is equal to zero.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the priority benefit of U.S. Prov. Pat.App. Ser. No. 61/806,152, filed on Mar. 28, 2013, which is owned by theassignee of the present application, and is incorporated herein byreference in its entirety.

INCORPORATION BY REFERENCE

The following patent application is incorporated herein by reference inits entirety:

“Multifiber Connectors for Multicore Optical Fiber Cables,” U.S. patentapplication Ser. No. 13/049,794, filed on Mar. 16, 2011, which is ownedby the assignee of the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of fiber optics,and in particular to structures for providing aligned connectorizationfor multicore optical fibers.

2. Background Art

Multicore fiber (MCF) technology has the potential of greatly increasingthe capacity of current optical fiber networks by allowing a pluralityof optical data signals to be carried in parallel by a single fiber.MCFs have been developed that have a diameter that is equal to, or closeto, that of a single-core fiber. The use of these MCFs in place ofsingle-core fibers increases patching densities without jeopardizingoptical performance or introducing fiber management issues.

One important technical issue to be addressed is connectorization, i.e.,how to suitably terminate MCF cables such that they exhibit anacceptably low level of insertion loss. MCFs currently under developmentmust have an insertion loss that is low enough to support datacommunication applications typically requiring 2 to 4 connections. Datacenter and enterprise structured cabling systems typically require amean insertion loss of 0.3 dB, with a standard deviation of 0.2 dB, andmaximum insertion loss of 0.75 dB.

SUMMARY OF INVENTION

An aspect of the invention is directed to a multicore fiber alignmentapparatus, comprising a chassis into which are mounted a number ofalignment components. Means are provided for holding a multicore fiberferrule having one or more capillaries extending therethrough.Fiber-holding means are provided for holding one or more multicorefibers in position to be mounted into the ferrule, such that eachmulticore fiber extends through a respective ferrule capillary. Therotation angle of each multicore fiber within its respective capillaryis monitored, relative to a reference rotational orientation. Means arefurther provided for rotating each of the multicore fibers within itsrespective capillary. The rotational orientation of each multicore fiberis fixed when its rotation angle is equal to zero, i.e., when itsrotational orientation is equal to the reference rotational orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an endface view of an exemplary multimode multicore fiberused to illustrate aspects of the invention.

FIG. 2 shows an isometric view of first and second multicore fibersegments to be connected together.

FIG. 3 shows a diagram illustrating first and second multicore fibersegments having different angles of rotation relative to a referencerotational orientation.

FIG. 4 shows a diagram illustrating an exemplary practice of a multicorefiber alignment apparatus according to an aspect of the invention.

FIG. 5 shows a diagram of an exemplary implementation of a fiber-holdingarrangement for use in the apparatus shown in FIG. 4.

FIG. 6 shows a diagram of an exemplary implementation of an indexingwheel assembly for use in the apparatus shown in FIG. 4.

FIGS. 7A-7C show a series of diagrams of alternative machine visionconfigurations for use in the apparatus shown in FIG. 4.

FIG. 8 shows a diagram of an exemplary implementation of a suction blockand suction pump for use in the FIG. 4 apparatus shown in FIG. 4.

FIG. 9 shows a flowchart of a general multicore fiber alignmenttechnique according to a further aspect of the invention.

DETAILED DESCRIPTION

Aspects of the present invention are directed to systems and techniquesfor providing aligned connectorization for multicore fibers.

The structures and techniques described herein take advantage of theMM-MCF design by increasing the volume of data transmitted by a singlefiber strand or, equivalently, reducing the number of fibers required toprovide a certain total bandwidth. To satisfy the demand for higherbandwidth, the current solution is to increase the number oftransmission fibers, which takes up more space and makes system breaksmore likely.

FIG. 1 shows an endface view of an exemplary multimode multicore fiber(MM-MCF) 10, described in detail in U.S. patent application Ser. No.13/045,064, which is owned by the assignee of the present applicationand which is incorporated herein by reference in its entirety.

Aspects of the invention are described with respect to MM-MCF 10.However, it will be appreciated that the present invention may also bepracticed using other types of MCFs, including MCFs having differentnumbers of cores, different types of cores or claddings, and differentcore configurations, which may or may not be symmetric with respect tothe fiber axis. Also, the MCF fiber could be twisted, such that each ofits cores forms a helix within the fiber around the fiber axis. If theconsidered core is centered on the fiber axis, i.e., if it is a centercore, the diameter of this helix is zero.

MM-MCF 10 comprises seven graded-index cores 110-116, produced fromseven graded-index LaserWave fiber core rods using a stack-and-drawprocess. The cores are arranged in a hexagonal array including a centercore 110 and six outer cores 111-116 that extend through a sharedcladding 12. The diameter of each of the cores is ˜27 μm, and the corepitch is ˜37 μm. The cladding diameter is 125 μm and the acrylate dualcoating layer (not shown) is 250 μm.

Although MCFs, such as the fiber 10 illustrated in FIGS. 1A and 1B, arenot yet commercially available, there are active development efforts tomake MCF commercially available in the near future because MCF is one ofthe solutions to address the need for increased data transmissioncapacity and reduced space for fiber optic cables and other transmissionmedia.

FIG. 2 is an isometric diagram illustrating the connection of a firstMM-MCF segment 10 a to a second MM-MCF segment 10 b. When connecting asegment of MM-MCF 10 to a second MM-MCF segment, care must be taken toensure precise alignment for respective pair of MM-MCF cores. Failure toachieve proper alignment can result in unacceptably high levels ofinsertion loss.

Positioning of a fiber having a single core is relatively easy becausethe single core is typically centered at the fiber axis. In such afiber, generally speaking, rotational orientation is irrelevant. Thesame is not true for an MCF. In MM-MCF 10, only one out of the sevencores is located at the fiber axis. Thus, in a given data transmissionapplication, most if not all of the active cores will be “satellite”cores, i.e., cores located away from the fiber axis.

For the purposes of the present description, the rotational orientationof a given segment of MM-MCF 10 is measured relative to arbitrarilypositioned x- and y-axes, with the MM-MCF segment centered at theorigin. The rotational orientation of the segment is quantified as theangle φ between the x-axis and an imaginary line 14 (FIG. 1) passingthrough the segment's center core 110 and two satellite cores 115 and112. It is noted that others techniques and conventions may be used todescribe rotational orientation.

FIG. 3 is a diagram illustrating a rotational misalignment between thecores at the endface of the first MM-MCF segment 10 a and the cores atthe endface of the second MM-MCF segment 10 b (shown in broken lines).In FIG. 3, the first MM-MCF segment 10 a is depicted as having arotational orientation in which φ_((a))=0. The second MM-MCF segment 10b is depicted as having a rotational orientation in which φ_((b))=10°.

It is noted that the respective endfaces of first and second multicorefibers to be connected to each other must have core configurations thatare mirror images of each other in order for all of the cores of thefirst fiber to be properly aligned with all of the cores of the secondfiber.

The cores need to be precisely clocked to mate with the correspondingcores of the mating connector. This requires a system able to recognizethe position of the cores before the alignment takes place and then ableto move the fiber to the proper position. The moving system has to beable to align cores with their nominal positions within a certain amountof tolerance (e.g., ±1°) to meet the connector loss requirement. Inother words, |φ_((a))|, |φ_((b))|≦1°, where |φ_((a))|, |φ_((b))| are themoduli (absolute values) of φ_((a)), φ_((b)).

The high number of cores to be aligned exacerbates the problem. Theconnector is rejected if even a single channel does not meet the lossrequirement. The channel loss is a function of the quality of thealignment of the cores. Manipulating individual fibers for multicorealignment is difficult given the 250 μm center-to-center pitch betweenadjacent fibers and the required alignment precision (no side-to-sidespace).

Further, an MCF may have a core configuration in which the outer coresof the MCF are helically twisted around the fiber's longitudinal axis. Atwisted core configuration makes pre-alignment difficult because therotational orientation of the cores at a cleaved endface depends uponits axial location.

An aspect of the invention is directed to a multifiber mechanicaltransfer (MT) connectorization system for MM-MCFs, in which avision-based approach is employed to achieve precise and repeatablepositioning of the transmitting channels in commercially availableMT-type optical connectors. The proper positioning of all fibers in anMT connector is required in order to meet the performance criteria ofthe connectorized assembly.

A multifiber MCF connector addresses the space issue in data centers byreducing the volume of fiber being used. At the same time, it will allowfor an increased volume of data transfer. MM-MCF 10 and theconnectorization system described herein are designed to bebackward-compatible with previous MT cable assembly deployments.

FIG. 4 shows a diagram of a multicore fiber alignment apparatus 40according to an aspect of the system. Apparatus 40 comprises a chassis41, to which the following components are mounted:

ferrule-holding means for holding a multicore fiber ferrule 42 havingone or more capillaries 421 extending therethrough;

fiber-holding means 43 for holding one or more multicore fibers 44 inposition to be mounted into the ferrule 42, such that each multicorefiber extends through a respective ferrule capillary 421;

an indexing wheel assembly 45, rotatable (as indicated by arrow A)around an axis that is parallel to the fiber axes, for rotating each ofthe multicore fibers 44 within its respective capillary 421;

a camera 46 and light source 461, or the like, for monitoring therotational orientation of each multicore fiber within its respectivecapillary, relative to a reference rotational orientation;

a suction block 47 and suction pump 48, or like means, for fixing therotational orientation of each multicore fiber 44 when its angularrotation is aligned with the reference rotational orientation, within aselected tolerance; and

a microprocessor controller 49.

In an exemplary practice of the invention, one or more MCFs 44 to beconnectorized are stripped and threaded through respective capillaries421 in MT ferrule 42 and held in place by fiber-holding means 43.

FIG. 5 shows an end view of an exemplary implementation of thefiber-holding means 43, comprising matching upper and lower blocks 431 aand 431 b. Each block is provided with a set of matching grooves 432 aand 432 b. When the upper and lower blocks are brought into contact witheach other, the matching grooves 432 a and 432 b form an array ofchannels that are shaped to closely receive a corresponding plurality ofMCFs.

Controller 49 (FIG. 4) rotates each MCF 44 by means of the indexingwheel assembly 45.

FIG. 6 shows a diagram of an exemplary implementation of an indexingwheel assembly 450, comprising an indexing wheel 451 that is rotated bya driving wheel 452. In the depicted implementation, the position of theindexing wheel assembly 450 is adjustable both up-and-down andside-to-side (indicated respectively by arrows B and C in FIG. 4).However, the axis of rotation of the indexing wheel is fixed in aparallel orientation relative to the fiber axis. Otherwise, turning theindexing wheel would apply axial force to the fiber and potentiallyshift the fiber along the groove in which it is seated.

Further, the plane spanned by the axis of the fiber currently beingaligned and the rotational axis of the indexing wheel should beperpendicular to the plane spanning the respective axes of all of thefibers in the MCF array. This arrangement prevents accidental rotationof two fibers at once.

Before a given MCF 44 is rotated, the indexing wheel assembly 450 ismoved such that the periphery of the driving wheel 452 comes in contactwith the fiber 44. After the fiber 44 has been rotated to the properposition and secured, the indexing wheel assembly 450 moves off andindexes to the next fiber.

Returning to FIG. 4: the position of the fiber 44 is verified duringrotation using a machine vision-based approach, in which, e.g., a camera46 and light source 461 are used to generate image data that istransmitted to controller 49, which uses the image data to determine therotational orientation of fiber 44. Controller 49 sends a signal to therotating wheel 45, instructing it to stop when the proper rotationalorientation of the fiber is achieved.

FIGS. 7A and 7B show alternative configurations of a one-camera machinevision subsystem for use in the system shown in FIG. 4. In theconfiguration shown in FIG. 7A, camera 46 is positioned transversely tothe fiber axis, such that it looks across the fiber axis, with lightsource 461 positioned opposite camera 46, on the other side of the MCF44. In the configuration shown in FIG. 7B, the camera 46 is positionedat a first cleaved fiber endface 441, such that it looks along the fiberaxis, with light source 461 positioned at a second fiber endface 442.

FIG. 7C shows a two-camera machine vision configuration in which firstand second cameras 46 a and 46 b are positioned transversely to thefiber axis, spaced 90 degrees apart (i.e., with the cameras positionedat ±45 degrees relative to vertical). The two-camera configuration isuseful with respect to a complicated MCF structure for detecting thecore profiles and making alignment more efficient and quick.

It is noted that other machine vision configurations that allow tomeasure the rotational orientation of the fibers can be used inconjunction with the practices of the invention described herein. Forexample, the radiation that is emitted by the light source or sourcesand detected by the camera or cameras does not necessarily have to belight in the visible wavelength range. Non-visible radiation, such asnear infrared or ultraviolet radiation, could also be used with suitabledetectors. For the purposes of the present description, it will beunderstood that the terms “visual” and “visually monitoring” are used torefer to light at all wavelengths that are detectible by a machinevision system, including light at wavelengths that are visible tohumans, as well as infrared and ultraviolet wavelengths.

FIG. 8 is a diagram of an exemplary implementation of a fiber-holdingsubassembly 50, comprising suction block 47 and suction pump 48. Thefunction of fiber-holding subassembly 50 is to selectably maintain therotational orientation of each MCF 44 after it has been rotationallyaligned by creating a differential between the atmospheric pressure atthe upper portion of the MCF and the suction-induced lower pressure atthe lower portion of the MCF. It is noted that fiber-holding subassembly50 may be implemented using other structures and techniques known in theart.

Suction block 47 comprises a plurality of grooves 471 that are shaped toreceive the lower halves of a corresponding array of MCFs 44. Eachgroove 471 includes one or more holes that provide an opening into arespective conduit 472 leading to suction pump 48. Suction block 47 andsuction pump 48 are configured such that suction is appliedindependently to each MCF 44. Thus, individual MCFs 44 can be held inplace in their respective grooves 471, while allowing the other MCFs 44to rotate freely.

As each MCF 44 is rotationally aligned, controller 49 fixes therotational orientation of the MCF 44 by sending a suitable signal to thesuction pump 48, which operates to create a pressure differentialbetween the atmospheric pressure at the top of the fiber and a lowerpressure at the bottom of the fiber.

Controller 49 rotates each MCF 44 one by one until it is rotationallyaligned with respect to the reference rotational orientation. Once eachfiber is rotationally aligned, suction block 47 maintains the rotationalorientation of the fiber as the indexing wheel assembly is moved to thenext fiber to be aligned. This process continues until all of the fibersin the connector are aligned. A suitable adhesive, such as epoxy, can becured in order to permanently fix the position of each MCF within itsrespective capillary.

In one practice of the invention, adhesive is applied in a two-stepprocess. In a first step, a quick-curing adhesive is applied to thefiber while it is still in the alignment jig. In a second step, aheat-curing adhesive, such as epoxy, is applied after the fiber has beenremoved from the alignment jig.

The proposed alignment method eliminates intermediary alignment stepsand therefore improves the quality of the alignment. Ferrule 42 becomesa part of the finished mating connector after all alignment steps arecompleted and the fibers are epoxied in place.

The information from the vision system employed in this method is usedto optimize the position of the fiber by controlling the rotation of theindexing wheel 45. Typically, it would utilize an intensity patternmatching software to determine the current position of the cores, i.e.,the angle of rotation of the fiber, relative to a prescribed referenceposition (i.e., angle of rotation). This reference orientation can bearbitrarily chosen, but is preferably the same for all fibers to beconnectorized. The software can use any algorithm or method that issuitable to determine the current angle difference between the fiber andthe reference orientation.

As an example, the current angle may be determined by least-squaresfitting the currently measured intensity pattern with a library ofintensity patterns that show the fiber for certain known rotation angles(e.g., 1-degree equispaced, relative to the reference orientation). Thecurrent angle is approximated by finding the rotation angle having alibrary intensity pattern that minimizes a selected norm (e.g., rootmean square) of the difference between the currently measured intensitypattern and each of the library intensity patterns. If necessary, afiner angular resolution can be achieved by interpolation and aniterative numerical algorithm such as the Levenberg-Marquardt method.

The indexing wheel needs to be rotated until the current rotation angle□ of the fiber is equal, or substantially equal, to zero, i.e., when thebest fit is given by the prescribed reference orientation.

As used herein, the term “substantially equal” refers to values that aresufficiently close to each other to achieve a desired result (e.g., anacceptably small amount of loss). For the purposes of the presentdescription, it will be understood that the term “equal to” is used torefer inclusively to values that are exactly equal to each other, aswell as to values that are substantially equal to each other.

In an exemplary practice of the invention, rotation angles are measuredin the interval [−180°, +180°] or [0°, 360°], with a targeted accuracyof 0.5 degrees.

If the indexing wheel and the fiber have the diameters D and d,respectively, and slippage between them can be neglected, then themodulus of the angle to be applied to the wheel is □d/D. However, due toimperfections such as backlash and torsion along the fiber, theresulting angle at the fiber tip may still be nonzero. Therefore, thedescribed procedure (determination of the current angle, then rotatingthe indexing wheel appropriately to null it) may have to be repeated afew times to meet the required angular tolerance.

Alternatively, the rotation angle □ of the fiber may also be trackedcontinuously (using the described fitting procedure or some othersuitable method) while the indexing wheel is turning. In this case, onlya stop signal needs to be sent to the indexing wheel when □=0 (withintolerance) is achieved.

It is further noted that, in the exemplary practice described above, theMCFs are rotated and monitored one at a time. It would also be possibleto practice the invention according to a technique in which two or moreMCFs, or all of the MCFs in a given array, are rotated and monitoredsimultaneously.

Although MCF core alignment in a MT connector is described herein, thepresent invention is equally applicable to other types of connectorssuch as LC, SC, ST, and FC. Also, the alignment device and methods ofaligning MCF cores within a connector equally work for a connector witha single MCF as well as a connector that accommodates a plurality ofMCFs as described here. Furthermore, the alignment device and methods ofaligning MCF cores within a connector equally work for single-mode MCFsas well. Even for those single-mode fibers that are supposed to beaxially symmetric, imperfections such as concentricity errors of thecores or the cladding within the coating can make the describedrotational orientation optimization useful.

FIG. 9 shows a flowchart illustrating a general multicore fiberalignment technique 90 according to the invention. Technique 90comprises the following steps:

91: Position one or more multicore fibers such that each multicore fiberextends through a respective capillary in a multicore fiber ferrule.

92: Visually monitor each multicore fiber to determine its angle ofrotation relative to a reference rotational orientation.

93: Rotate each multicore fiber within its respective capillary until itachieves an aligned rotational orientation, wherein the angle ofrotation of the fiber relative to the reference rotational orientationequals zero, within tolerance.

94: Fix the rotational orientation of each multicore fiber when itsangular rotation is in alignment with the reference rotationalorientation, within a selected tolerance.

While the foregoing description includes details which will enable thoseskilled in the art to practice the invention, it should be recognizedthat the description is illustrative in nature and that manymodifications and variations thereof will be apparent to those skilledin the art having the benefit of these teachings. It is accordinglyintended that the invention herein be defined solely by the claimsappended hereto and that the claims be interpreted as broadly aspermitted by the prior art.

What is claimed is:
 1. A multicore fiber alignment method, comprising:(a) positioning one or more multicore fibers such that each multicorefiber extends through a respective capillary in a multicore fiberferrule; (b) visually monitoring each multicore fiber to determine itsangle of rotation relative to a reference rotational orientation; (c)rotating each multicore fiber within its respective capillary until itachieves an aligned rotational orientation, wherein the angle ofrotation of the fiber relative to the reference rotational orientationequals zero, within tolerance; and (d) fixing the rotational orientationof each multicore fiber when its angular rotation is in alignment withthe reference rotational orientation, within a selected tolerance. 2.The method of claim 1, wherein step (b) comprises utilizing intensitypattern matching to determine the current rotational orientation of thefiber cores relative to the reference orientation.
 3. The method ofclaim 2, wherein the current rotational orientation of the fiber coresis determined by comparing the currently measured intensity pattern witha library of intensity patterns for known rotation angles.
 4. The methodof claim 3, wherein the current rotation angle is approximated by therotation angle of the particular library intensity pattern thatminimizes a norm of the difference between the currently measuredintensity pattern and all library intensity patterns.
 5. The method ofclaim 4, wherein a finer angular resolution can be achieved byinterpolation and an iterative numerical algorithm.
 6. The method ofclaim 5, wherein the known rotation angles are equispaced relative tothe reference orientation.
 7. The method of claim 1, wherein step (c)includes using an indexing wheel to rotate each of the multicore fibers.8. The method of claim 7, wherein step (c) includes using a drivingwheel to rotate the indexing wheel.
 9. The method of claim 1, whereinstep (c) includes adjusting the position of the indexing wheel, suchthat when each multicore fiber is rotated, the axis of rotation of theindexing wheel is parallel to the multicore fiber.
 10. The method ofclaim 1, where steps (b) and (c) are repeated to achieve a requiredangular tolerance.
 11. The method of claim 1, wherein the rotation angleof the fiber is monitored continuously while the fiber is being rotated,and wherein the rotation of the wheel is stopped when the rotation angleis equal to zero, within tolerance.
 12. The method of claim 1, whereinstep (d) includes using a suction assembly to maintain the rotationalorientation of each individual multicore fiber.
 13. The method of claim12, wherein, in step (d), the suction assembly comprises a block with anarray of channels corresponding to the one or more multicore fibers,wherein each channel is provided with one or more conduit openings forapplying a suction to a fiber seated within the channel.
 14. The methodof claim 1, wherein step (d) includes maintaining the rotationalorientation of each multicore fiber until an epoxy applied to themulticore fiber is cured so as to permanently fix its rotationalorientation.